[GRASSLIST:78] Re: interpolated values out of range

[Markus Neteler]

On Tue, Mar 14, 2006 at 12:03:06PM +0100, Stefan Istvan wrote:
> Hello List!
> 
> I have some vector points with a value attached to them, and I would
> like to interpolate a raster surface from them. 
> My problem is that the resulting raster map contains values that are
> smaller then the minimum value of my vector points.
> I set the smooth paramter to 0, in order to the resulting surface cross
> exactly the vector points.
> I also set the tension paramter to 180 because during the interpolation
> grass said that overshoot occures and suggested to increase tension.
> The imported vector point values range from 10-35900, but on the
> resulting raster map there are values far below zero (-2000).
> I also tried so set smooth to default (0.1), but still get values below
> 0.
> Is there any method to calculate the value of tension and smooth, or
> just trying?

In GRASS 6.1-CVS, cross validation was added as flag to optimize
tension and smoothing.

See: 
 http://grass.itc.it/grass61/manuals/html61_user/v.surf.rst.html
 -> Cross validation procedure

Markus